Luminescent pigments and foils with color-shifting properties

ABSTRACT

Interference pigment flakes and foils are provided which have luminescent and color-shifting properties. The pigment flakes can have a symmetrical coating structure on opposing sides of a core layer, can have an asymmetrical coating structure with all of the layers on one side of the core layer, or can be formed with encapsulating coatings around the core layer. The coating structure of the flakes and foils includes a core layer, a dielectric layer overlying the core layer, and an absorber layer overlying the dielectric layer. A luminescent material is incorporated into the flakes or foils as a separate layer or as at least part of one or more of the other layers. The pigment flakes and foils exhibit a discrete color shift so as to have a first color at a first angle of incident light or viewing and a second color different from the first color at a second angle of incident light or viewing. The pigment flakes can be interspersed into liquid media such as paints or inks to produce colorant materials for subsequent application to objects or papers. The foils can be laminated to various objects or can be formed on a carrier substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to luminescent color-shiftingpigments and foils. More particularly, the present invention relates tomultilayer color-shifting pigment flakes and foils which haveluminescent materials incorporated therein.

2. Background Technology

Various color-shifting pigments, colorants, and foils have beendeveloped for a wide variety of applications. For example,color-shifting pigments have been used in applications such ascosmetics, inks, coating materials, ornaments, ceramics, automobilepaints, anti-counterfeiting hot stamps and anti-counterfeiting inks forsecurity documents and currency. Such pigments, colorants, and foilsexhibit the property of changing color upon variation of the angle ofincident light, or as the viewing angle of the observer is shifted.

The color-shifting properties of the pigments and foils can becontrolled through proper design of the optical thin films ororientation of the molecular species used to form the flake or foilcoating structure. Desired effects can be achieved through the variationof parameters such as thickness of the layers forming the flakes andfoils and the index of refraction of each layer. The changes inperceived color which occur for different viewing angles or angles ofincident light are a result of a combination of selective absorption ofthe materials comprising the layers and wavelength dependentinterference effects. The interference effects, which arise from thesuperposition of light waves that have undergone multiple reflections,are responsible for the shifts in color perceived with different angles.The reflection maxima changes in position and intensity, as the viewingangle changes, due to the absorption characteristics of a material whichare selectively enhanced at particular wavelengths from the interferencephenomena.

Various approaches have been used to achieve such color-shiftingeffects. For example, small multilayer flakes, typically composed ofmultiple layers of thin films, are dispersed throughout a medium such aspaint or ink that may then be subsequently applied to the surface of anobject. Such flakes may optionally be overcoated to achieve desiredcolors and optical effects. Another approach is to encapsulate smallmetallic or silicatic substrates with varying layers and then dispersethe encapsulated substrates throughout a medium such as paint or ink.Additionally, foils composed of multiple layers of thin films on asubstrate material have been made.

One manner of producing a multilayer thin film structure is by formingit on a flexible web material with a release layer thereon. The variouslayers are deposited on the web by methods well known in the art offorming thin coating structures, such as PVD, sputtering, or the like.The multilayer thin film structure is then removed from the web materialas thin film color-shifting flakes, which can be added to a polymericmedium such as various pigment vehicles for use as an ink or paint. Inaddition to the color-shifting flakes, additives can be added to theinks or paints to obtain desired color-shifting results.

Color-shifting pigments or foils are formed from a multilayer thin filmstructure that includes the same basic layers. These include an absorberlayer(s), a dielectric layer(s), and optionally a reflector layer, invarying layer orders. The coatings can be formed to have a symmetricalmultilayer thin film structure, such as:

absorber/dielectric /reflector/dielectric/absorber; or

absorber/dielectric/absorber.

Coatings can also be formed to have an asymmetrical multilayer thin filmstructure, such as:

absorber/dielectric/reflector.

For example, U.S. Pat. No. 5,135,812 to Phillips et al., which isincorporated by reference herein, discloses color-shifting thin filmflakes having several different configurations of layers such astransparent dielectric and semi-transparent metallic layered stacks. InU.S. Pat. No. 5,278,590 to Phillips et al., which is incorporated byreference herein, a symmetric three layer optical interference coatingis disclosed which comprises first and second partially transmittingabsorber layers which have essentially the same material and thickness,and a dielectric spacer layer located between the first and secondabsorber layers.

Color-shifting platelets for use in paints are disclosed in U.S. Pat.No. 5,571,624 to Phillips et al., which is incorporated by referenceherein. These platelets are formed from a symmetrical multilayer thinfilm structure in which a first semi-opaque layer such as chromium isformed on a substrate, with a first dielectric layer formed on the firstsemi-opaque layer. An opaque reflecting metal layer such as aluminum isformed on the first dielectric layer, followed by a second dielectriclayer of the same material and thickness as the first dielectric layer.A second semi-opaque layer of the same material and thickness as thefirst semi-opaque layer is formed on the second dielectric layer.

As discussed above, there are a wide variety of thin film devicesproduced today, including many that are used as security devices.Although color-shifting pigments and foils provide properties that makethem extremely useful as components of security devices, it is desirableto seek additional levels of security by adding additional features.

In European patent application publication EP 0927749A1 to Bleikolm etal. (hereafter “Bleikolm”) multilayered thin films for security andanti-counterfeiting uses are disclosed. Two or more thin layers aredeposited in a film, which is subsequently ground into thin filmparticles. These particles can be mixed into a coating material orincorporated into a bulk material and are optionally luminescent. Boththe sequence of layers and their thicknesses can be used to analyze andidentify the particles. Bleikolm further discloses the use of themultilayer thin film structure as a tag. Further, the thin filmparticles can be used in a mixture with color-shifting pigments toprovide an ink with increased properties. Nevertheless, the thin filmparticles do not themselves have color-shifting properties.

European Patent Application Publication EP 0927750A1 to Rozumek et al.(hereafter “Rozumek”) discloses the use of two distinct inorganicchemicals incorporated into particles in a predefined and analyticalratio. The particles can be mixed into a coating material orincorporated into a bulk material. The particles provide both spatialand chemical information for security and anti-counterfeitingapplications based on the material of the particles and their physicallocation in an ink as applied to a surface. In one embodiment, one orboth of the particles are luminescent.

Unfortunately, the performance of prior color-shifting/luminescent inkshas several drawbacks. For example, when color-shifting flakes arecombined with luminescent particles, separation tends to occur. Thecolor-shifting flakes and luminescent particles also tend to beincompatible with the same ink or coating vehicle, making them difficultto use together. Further, the luminescent particles tend to opacify anddull the color performance of the color-shifting flakes.

Additionally, the simple physical mixing of separate color-shifting andluminescent species does not allow for control of the re-emittedspectrum at differing angles since there is no way to control theoptical path within simple physical mixtures. Finally, in the currentstate of the art, forming a thin film interference coating structurethat employs a luminescent material as the dielectric is impracticalbecause the stoichiometry of inorganic luminescent materials is veryimportant and their production usually requires processing attemperatures higher than standard coating temperatures.

Accordingly, there is a need for improved coating structures and methodsthat avoid the above difficulties in forming luminescent color-shiftingcompositions.

SUMMARY AND OBJECTS OF THE INVENTION

It is an object of the present invention to provide pigments and foilsfor use as or part of security devices that have both overt and covertfeatures.

It is another object of the invention to provide new features forcolor-shifting pigments and foils that increase their value for use assecurity devices.

It is a further object of the invention to provide luminescentcolor-shifting pigments that do not separate into discrete components.

It is another object of the invention to provide a luminescentcolor-shifting pigment, wherein the pigment's luster is not diminishedby the presence of the luminescent material.

To achieve the forgoing objects and in accordance with the invention asembodied and broadly described herein, interference pigment flakes andfoils are provided that have luminescent and color-shifting properties.The pigment flakes can assume a variety of useful configurations. Forexample, a flake may have a symmetrical coating structure on opposingsides of a core layer, or an asymmetrical coating structure with all ofthe layers on one side of the core layer, or can be formed withencapsulating coatings around the core layer, or other configurations.The coating structure of the flakes and foils generally includes a corelayer such as a reflector layer or transparent particle, a dielectriclayer overlying the core layer, and an absorber layer overlying thedielectric layer.

A luminescent material is incorporated into the flakes or foils as aseparate layer or as at least part of one or more of the other layers.The luminescent material can be a fluorescent material, a phosphorescentmaterial, an electroluminescent material, a chemoluminescent material, atriboluminescent material, or other like materials. Such luminescentmaterials exhibit a characteristic emission of electromagnetic energy inresponse to an energy source generally without any substantial rise intemperature.

The luminescent pigment flakes and foils exhibit a discrete color shiftso as to have a first color at a first angle of incident light orviewing angle and a second color different from the first color at asecond angle of incident light or viewing. The pigment flakes can beinterspersed into liquid media such as paints or inks to producecolorant materials for subsequent application to objects or papers.Another embodiment of the invention comprises a mixture of one type ofluminescent color-shifting flakes with another type of luminescentand/or non-luminescent color-shifting flakes in a predetermined ratio.The foils can be laminated to various objects or can be formed on acarrier substrate.

The foregoing objects and features of the present invention will becomemore fully apparent from the following description and appended claims,or may be learned by the practice of the invention as set forthhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to illustrate the manner in which the above-recited and otheradvantages and objects of the invention are obtained, a more particulardescription of the invention briefly described above will be rendered byreference to specific embodiments thereof which are illustrated in theappended drawings. Understanding that these drawings depict only typicalembodiments of the invention and are not therefore to be consideredlimiting of its scope, the invention will be described and explainedwith additional specificity and detail through the use of theaccompanying drawings in which:

FIG. 1 is a schematic representation of a luminescent color-shiftingstructure according to the invention;

FIG. 2 is a plot demonstrating one position of a luminescent layerrelative to electric field intensity in a luminescent color-shiftingstructure according to the invention;

FIG. 3 is a plot demonstrating another position of a luminescent layerrelative to electric field intensity in a luminescent color-shiftingstructure according to the invention;

FIG. 4 is a schematic representation of another luminescentcolor-shifting structure according to the invention;

FIG. 5 is a schematic representation of yet another luminescentcolor-shifting structure according to the invention;

FIG. 6 is a schematic representation of yet another luminescentcolor-shifting structure according to the invention;

FIG. 7 is a schematic representation of another luminescentcolor-shifting structure according to the invention;

FIG. 8 is a schematic representation of yet another luminescentcolor-shifting structure according to the invention;

FIG. 9 is a schematic representation of yet another luminescentcolor-shifting structure according to the invention;

FIG. 10 is a schematic representation of another luminescentcolor-shifting structure according to the invention;

FIG. 11 is a schematic representation of yet another luminescentcolor-shifting structure according to the invention;

FIG. 12 is a schematic representation of another luminescentcolor-shifting structure according to the invention;

FIG. 13 is a schematic representation of yet another luminescentcolor-shifting structure according to the invention;

FIG. 14 is a plot demonstrating characteristic absorption of aluminescent material relative to various angles of incidentelectromagnetic energy; and

FIG. 15 is a plot demonstrating characteristic angle-sensitive emissionof a luminescent color-shifting structure according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed generally to luminescent multilayercolor-shifting pigments and foils and methods of making the same. Thepigment flakes and foils have substantial shifts in chroma and hue withchanges in angle of incident light or viewing angle of an observer. Suchan optical effect, sometimes known as color-shifting, opticalvariability, or goniochromaticity, allows a perceived color to vary withthe angle of illumination or observation. Accordingly, the pigmentflakes and foils exhibit a first color at a first angle of incidentlight or viewing angle and a second color different from the first colorat a second angle of incident light or viewing.

Generally, the luminescent color-shifting pigment flakes of theinvention can have a symmetrical coating structure on opposing sides ofa core layer, can have an asymmetrical coating structure with all of thelayers on one side of the core layer, or can be formed withencapsulating coatings which surround a core layer. The flakes and foilsgenerally have a thin film structure that includes a core layer, adielectric layer overlying the core layer, and an absorber layeroverlying the dielectric layer. Each of these layers in the coatingstructures of the flakes and foils of the invention will be discussed infurther detail hereinafter. A luminescent material is incorporated intoone or more of the layers of the flakes or foils, with the one or morelayers being partially or completely composed of the luminescentmaterial.

In one embodiment of the invention, one or more of the thin film layersis comprised of a luminescent material having optical properties, suchas index of refraction and extinction coefficient, which contribute tothe creation of a color-shifting effect as well as providing luminescentproperties to the flake or foil. For example, luminescent materials thatare non-absorbing in the visible spectrum could be used as dielectricswhereas absorbing luminescent materials could be used as absorbers,reflectors, or partially absorbing dielectrics such as lossydielectrics.

Another embodiment of the invention uses a distinct luminescent layerwithin the multilayer stack. This approach allows for additional controlof the optical path of incident light and thereby control of the anglesof excitation and refraction. Thus, for example, a distinct luminescentsublayer can be interposed at a certain thickness between two dielectricsublayers. The three sublayers together function as a dielectric layerwhile the luminescent sublayer provides the luminescence to the flake orfoil.

Methods of incorporating luminescent materials and layers in an opticalstack of a thin film flake or foil generally include sol-gel methods,use of nanoreactors, organic polymer coating processes, vacuumdeposition processes, or hybrid combinations of the above methods.

The luminescent pigment flakes can be interspersed into liquid mediasuch as paints or inks to produce various color-shifting colorantcompositions for subsequent application to objects or papers. Theluminescent foils can be laminated to various objects or can be formedon a carrier substrate. The present invention also includes a mixture ofluminescent color-shifting flakes and non-luminescent color-shiftingflakes in a predetermined ratio.

As used herein, the term “luminescent material” refers to any atomic ormolecular species or solid-state compound that converts at least part ofincident energy into emitted electromagnetic radiation with acharacteristic signature. Nonlimiting examples include luminescentmaterials that exhibit fluorescence, phosphorescence, and the like.These materials can be incorporated into one or more of the layers thatmake up the flake or foil structure. The luminescent material can beemployed in solid solution form in the flake or foil, or can be a solidphase such as a crystalline phosphor material.

The function of the luminescent material is to impart optical stimuliresponsive characteristics to the pigment flakes and foils. For example,when the pigment flakes or foils are illuminated with electromagneticradiation, bombarded with ionizing particles or radiation, or exposed toother excitation energy sources, the flakes or foils emit ultraviolet,visible or infrared radiation of a characteristic wavelength associatedwith the luminescent material species, or the optical interferencecharacteristics of the flake. Numerous types of luminescent materialspecies are known to one having ordinary skill in the art ofphotochemistry and physics, and may produce emission of light throughany of the emission processes, such as single-photon emission, multiplephoton emission, and the like. Examples of suitable luminescentmaterials for use in the present invention are described in furtherdetail hereafter.

The non-luminescent layers in the color-shifting flakes and foils of theinvention can be formed using conventional thin film depositiontechniques, which are well known in the art of forming thin film coatingstructures. Nonlimiting examples of such thin film deposition techniquesinclude physical vapor deposition (PVD), chemical vapor deposition(CVD), plasma enhanced (PE) variations thereof such as PECVD ordownstream PECVD, sputtering, electrolysis deposition, and otherdeposition methods capable of forming discrete and uniform thin filmlayers. The luminescent materials can be incorporated into the preformedflake or foil structures by a variety of novel methods which arediscussed in further detail hereafter.

The present inventors have discovered that color-shifting pigments andfoils having luminescent materials incorporated therein producesurprising results. For example, it is an unexpected result of theinvention that the addition of certain luminescent materials tocolor-shifting pigments does not degrade the performance of thecolor-shifting pigments. Rather, luminescence becomes a secondaryfeature of the pigments that functions independently of the color shift.In addition, the luminescence yield is surprisingly much higher forcolor-shifting pigment flakes having luminescent materials therein thanfor mixtures of color-shifting pigment flakes and fluorescent particles.This is due to a higher percentage of luminescence material available tolight exposure in the luminescent pigment flakes than in the mixture ofcolor-shifting pigments and luminescent particles.

Another surprising result occurs when the luminescent layer thicknessallows for a high percentage of excitation wavelength light to passthrough to an underlying layer structure of a pigment flake or foil. Theexcitation light is reflected from the underlying layer structure backinto the luminescent layer allowing the luminescent layer to absorb moreenergy and thereby increase the luminescence yield. This phenomenon ismanifested as a change in luminescence intensity with angle ofexcitation light, since the reflected excitation light is subject to thesame incident angle dependency rules as visible light.

A further novel feature of the present invention is the ease of handlingand reliability of the luminescent color-shifting pigments. Conventionalmixtures of color-shifting pigments and luminescent particles result inmixtures with a tendency to separate into individual components. Thus,the presence of the luminescent material is more easily detectable andit is more difficult to ensure consistent dispersions. The presentinvention overcomes these problems because the luminescent materialcannot separate from the pigment flakes, ensuring uniform compositions.In addition, when luminescent dyes are incorporated into thecolor-shifting pigment flakes, there is no deleterious effect on theability of the color-shifting pigment flakes to be substantially planarwhen set.

Advantageously, the combined luminescent material and interferencelayers also make a structural analysis of the pigment flakes moredifficult for potential counterfeiters. While the luminescent effectsare detectable, the structure that creates the luminescent effectscannot be observed by microscopic techniques. It is thus more difficultto analyze and emulate the pigment flakes. In contrast, mixtures ofluminescent particles and color-shifting pigments can be readily studiedunder a microscope to isolate and identify the luminescent particles.

In one embodiment of the present invention, the luminescent materialemits electromagnetic radiation when illuminated with electromagneticenergy containing the excitation wavelengths of the luminescentmaterial. The emission of electromagnetic radiation from thecolor-shifting pigment or foil is a function of the luminescentmaterial's composition and concentration, the incident energy, theoverall design of the thin film stack in the flake or foil, theplacement of the luminescent layer(s) within the stack, the angle ofincidence, and the wavelength-dependant electric field intensityreaching the luminescent layer.

Referring now to the drawings, wherein like structures are provided withlike reference designations, the drawings only show the structuresnecessary to understand the present invention. FIG. 1 depicts aluminescent color-shifting pigment flake 20 according to one embodimentof the invention. The flake 20 is a five-layer design having a generallysymmetrical multilayer thin film structure on opposing sides of areflector layer 22. Thus, first and second dielectric layers 24 a and 24b are disposed respectively on opposing sides of reflector layer 22, andfirst and second absorber layers 26 a and 26 b are disposed respectivelyon each of dielectric layers 24 and 25. It is a feature of the inventionthat at least one of the above layers is a luminescent material, orincludes a luminescent material as a sublayer or dispersed throughoutthe layer. Thus, the luminescent material can be present in thereflector layer, dielectric layer, or absorber layer, depending on thedesired structure and material of the particular pigment. Alternatively,the luminescent material can be selected so as to comprise the entirereflector, dielectric, or absorber layer. Each of these layers in thecoating structure of flake 20 is discussed below in greater detail.

Although not illustrated, flake 20 can also include further opticalcoatings. For example, an outer luminescent coating layer could beformed on flake 20. Such a luminescent coating structure for pigmentflakes and foils is disclosed in copending U.S. application Ser. No.09/715,934, filed on Nov. 17, 2000, the disclosure of which isincorporated by reference herein. Thus, a luminescent flake or foil ofthe invention can include a luminescent material incorporated therein aswell as an outer luminescent coating layer. Of course, one skilled inthe art will recognize that various other optical coatings can be usedas long as they do not excessively interfere with the color-shifting orluminescent properties of the flake or foil.

The reflector layer 22 of flake 20 can be composed of various materials.Presently preferred materials are one or more metals, one or more metalalloys, or combinations thereof, because of their high reflectivity andease of use, although non-metallic reflective materials could also beused. Nonlimiting examples of suitable metallic materials for thereflector layer include aluminum, silver, copper, gold, platinum, tin,titanium, palladium, nickel, cobalt, rhodium, niobium, chromium, andcombinations or alloys thereof. These can be selected based on the coloreffects desired. The reflector layer can be formed to have a suitablephysical thickness of from about 200 angstroms (Å) to about 10,000 Å,and preferably from about 400 Å to about 700 Å.

The dielectric layers 24 a and 24 b act as spacers in the thin filmstack structure of flake 20. The dielectric layers are formed to have aneffective optical thickness for imparting interference color and desiredcolor-shifting properties. The dielectric layers may be optionallyclear, or may be selectively absorbing so as to contribute to the coloreffect of a pigment. The optical thickness is a well known opticalparameter defined as the product ηd, where η is the refractive index ofthe layer and d is the physical thickness of the layer. Typically, theoptical thickness of a layer is expressed in terms of a quarter waveoptical thickness (QWOT) that is equal to 4ηd/λ, where λ is thewavelength at which a QWOT condition occurs. The optical thickness ofdielectric layers can range from about 2 QWOT at a design wavelength ofabout 400 nm to about 9 QWOT at a design wavelength of about 700 nm, andpreferably 2-6 QWOT at 400-700 nm, depending upon the color shiftdesired. The dielectric layers typically have a physical thickness ofabout 100 nm to about 800 nm.

Suitable materials for dielectric layers include those having a “high”index of refraction, defined herein as greater than about 1.65, as wellas those have a “low” index of refraction, which is defined herein asabout 1.65 or less. Each of the dielectric layers can be formed of asingle material or with a variety of material combinations andconfigurations. For example, the dielectric layers can be formed of onlya low index material or only a high index material, a mixture ormultiple sublayers of two or more low index materials, a mixture ormultiple sublayers of two or more high index materials, or a mixture ormultiple sublayers of low index and high index materials. In addition,the dielectric layers can be formed partially or entirely of high/lowdielectric optical stacks, which are discussed in further detail below.When a dielectric layer is formed partially with a dielectric opticalstack, the remaining portion of the dielectric layer can be formed witha single material or various material combinations and configurations asdescribed above.

Examples of suitable high refractive index materials for the dielectriclayer include zinc sulfide (ZnS), zinc oxide (ZnO), zirconium oxide(ZrO₂), titanium dioxide (TiO₂), carbon (C), indium oxide (In₂O₃),indium-tin-oxide (ITO), tantalum pentoxide (Ta2O5), ceric oxide (CeO₂),yttrium oxide (Y₂O₃), europium oxide (Eu₂ 0 ₃), iron oxides such as(II)diiron(III) oxide (Fe₃O₄) and ferric oxide (Fe₂O₃), hafnium nitride(HfN), hafnium carbide (HfC), hafnium oxide (HfO₂), lanthanum oxide(La₂O₃), magnesium oxide (MgO), neodymium oxide (Nd₂O₃), praseodymiumoxide (Pr₆O₁₁), samarium oxide (Sm₂O₃), antimony trioxide (Sb₂O₃),silicon carbide (SiC), silicon nitride (Si₃N₄), silicon monoxide (SiO),selenium trioxide (Se₂O₃), tin oxide (SnO₂), tungsten trioxide (WO₃),combinations thereof, and the like.

Suitable low refractive index materials for the dielectric layer includesilicon dioxide (SiO₂), aluminum oxide (Al₂O₃), metal fluorides such asmagnesium fluoride (MgF₂), aluminum fluoride (AlF₃), cerium fluoride(CeF₃), lanthanum fluoride (LaF₃), sodium aluminum fluorides (e.g.,Na₃AlF₆ or Na₅Al₃F₁₄), neodymium fluoride (NdF₃), samarium fluoride(SmF₃), barium fluoride (BaF₂), calcium fluoride (CaF₂), lithiumfluoride (LiF), combinations thereof, or any other low index materialhaving an index of refraction of about 1.65 or less. For example,organic monomers and polymers can be utilized as low index materials,including dienes or alkenes such as acrylates (e.g., methacrylate),perfluoroalkenes, polytetrafluoroethylene (Teflon), fluorinated ethylenepropylene (FEP), combinations thereof, and the like.

It should be appreciated that several of the above-listed dielectricmaterials are typically present in non-stoichiometric forms, oftendepending upon the specific method used to deposit the dielectricmaterial as a coating layer, and that the above-listed compound namesindicate the approximate stoichiometry. For example, silicon monoxideand silicon dioxide have nominal 1:1 and 1:2 silicon:oxygen ratios,respectively, but the actual silicon:oxygen ratio of a particulardielectric coating layer varies somewhat from these nominal values. Suchnon-stoichiometric dielectric materials are also within the scope of thepresent invention.

As mentioned above, the dielectric layers can be formed of high/lowdielectric optical stacks, which have alternating layers of low index(L) and high index (H) materials. When a dielectric layer is formed of ahigh/low dielectric stack, the color shift at angle will depend on thecombined refractive index of the layers in the stack. Examples ofsuitable stack configurations for the dielectric layers include LH, HL,LHL, HLH, HLHL, LHLH, as well as various multiples and combinationsthereof. In these stacks, LH, for example, indicates discrete layers ofa low index material and a high index material. In an alternativeembodiment, the high/low dielectric stacks are formed with a gradientindex of refraction. For example, the stack can be formed with layershaving a graded index low-to-high, a graded index high-to-low, a gradedindex low-to-high-to-low, a graded index high-to-low-to-high, as well ascombinations and multiples thereof. The graded index is produced by agradual variance in the refractive index, such as low-to-high index orhigh-to-low index, of adjacent layers. The graded index of the layerscan be produced by changing gases during deposition or co-depositing twomaterials (e.g., L and H) in differing proportions. Various high/lowoptical stacks can be used to enhance color-shifting performance,provide antireflective properties to the dielectric layer, and changethe possible color space of the pigments of the invention.

The dielectric layers can each be composed of the same material or adifferent material, and can have the same or different optical orphysical thickness for each layer. It will be appreciated that when thedielectric layers are composed of different materials or have differentthicknesses, the flakes exhibit different colors on each side thereofand the resulting mix of flakes in a pigment or paint mixture would showa new color which is the combination of the two colors. The resultingcolor would be based on additive color theory of the two colors comingfrom the two sides of the flakes. In a multiplicity of flakes, theresulting color would be the additive sum of the two colors resultingfrom the random distribution of flakes having different sides orientedtoward the observer.

The absorber layers 26 a and 26 b of flake 20 can be composed of anyabsorber material having the desired absorption properties, includingboth selective absorbing materials and nonselective absorbing materials.For example, the absorber layers can be formed of nonselective absorbingmetallic materials deposited to a thickness at which the absorber layeris at least partially absorbing, or semi-opaque. Nonlimiting examples ofsuitable absorber materials include metallic absorbers such as chromium,aluminum, nickel, palladium, platinum, titanium, vanadium, cobalt, iron,tin, tungsten, molybdenum, rhodium, niobium, as well as other absorberssuch as carbon, graphite, silicon, germanium, cermet, ferric oxide orother metal oxides, metals mixed in a dielectric matrix, and othersubstances that are capable of acting as a uniform or selective absorberin the visible spectrum. Various combinations, mixtures, compounds, oralloys of the above absorber materials may be used to form the absorberlayers of flake 20.

Examples of suitable alloys of the above absorber materials includeInconel (Ni—Cr—Fe), and titanium-based alloys, such as titanium mixedwith carbon (Ti/C), titanium mixed with tungsten (Ti/W), titanium mixedwith niobium (Ti/Nb), and titanium mixed with silicon (Ti/Si), andcombinations thereof. Examples of suitable compounds for the absorberlayers include titanium-based compounds such as titanium nitride (TiN),titanium oxynitride (TiN_(x)O_(y)), titanium carbide (TiC), titaniumnitride carbide (TiN_(x)C_(y)), titanium oxynitride carbide(TiN_(x)O_(y)C_(z)), titanium silicide (TiSi₂), titanium boride (TiB₂),and combinations thereof. In the case of TiN_(x)O_(y) andTiN_(x)O_(y)C_(z), preferably x=0 to 1, y=0 to 1, and z=0 to 1, wherex+y=1 in TiN_(x)O_(y) and x+y+z=1 in TiN_(x)O_(y)C_(z). ForTiN_(x)C_(z), preferably x=0 to 1 and z=0 to 1, where x+z=1.Alternatively, the absorber layers can be composed of a titanium-basedalloy disposed in a matrix of Ti, or can be composed of Ti disposed in amatrix of a titanium-based alloy.

The absorber layers are formed to have a physical thickness in the rangefrom about 30 Å to about 500 Å, and preferably about 100 Å to about 175Å, depending upon the optical constants of the absorber layer materialand the desired peak shift. The absorber layers can each be composed ofthe same material or a different material, and can have the same ordifferent physical thickness for each layer.

The luminescent material incorporated into flake 20 can be composed ofeither an organic or inorganic material which has the property ofluminescence. In general, luminescence is the emission ofelectromagnetic radiation, or light, from a material without anyassociated change in temperature, resulting from such causes as chemicalreactions, electron bombardment, electromagnetic radiation, and electricfields. Many luminescent materials are excited by high-energy photons orelectrons, absorbing incident electromagnetic radiation in onewavelength range, and emitting electromagnetic radiation in another.Luminescence is typically subdivided into the subcategories offluorescence and phosphorescence. Fluorescence occurs where a substanceemits electromagnetic radiation while absorbing some form of energy,with the emission ceasing abruptly when the input energy ceases.Phosphorescence occurs where a substance emits light following theabsorption of energy, with the emission continuing for a relatively longtime after the energy input has ceased. Additional subcategories ofluminescence include polarization of the incident electromagneticradiation, and non-linear optical effects such as second harmonicgeneration.

The luminescent materials used in the present invention can be excitedby various energy sources such as infrared radiation, ultravioletradiation, visible light, electric fields (electroluminescence),magnetic fields (magnetoluminescence), chemical reaction(chemoluminescence), as well as by mechanical stress(triboluminescence).

Nonlimiting examples of suitable organic luminescent materials includefluorescent dyes such as those in the coumarin class, the xanthaneclass, the acridine class, and numerous others as known to those skilledin the art. Specific examples include Dansyl, prodene, fluorescene,rhodamine, and the like. A fluorescent dye can also be combined with aliquid crystal polymer for incorporation into the color-shifting pigmentflakes or foils. Other suitable luminescent materials that can be usedin the present invention include the dyes in U.S. Pat. Nos. 4,173,002,5,329,540, and 5,912,257, the disclosures of which are incorporated byreference herein.

Suitable inorganic luminescent materials for use in the inventioninclude halophosphate phosphors, phosphate phosphors, silicatephosphors, aluminate phosphors, borate phosphors, tungstate phosphors,lanthanide phosphors, and the like.

Other luminescent materials useful in the invention includeelectroluminescent materials such as ZnS, Mn⁺⁺, ZnS:TbF₃, andpi-conjugated polymers; chemoluminescent materials such as dioxetanes,and acridinium salts; and second harmonic generators such asnitrogen-substituted amine stilbene derivatives, molecular complexes ofSbI₃ and sulfur, non-centrosymmetric dye aggregates. The luminescentmaterial may also be composed of solid phase or water-soluble quantumdot particles, such as are disclosed in PCT Publication No. WO 00/29617.Such quantum dot particles comprise a core, a cap and a hydrophilicattachment group. The “core” is a nanoparticle-sized semiconductor.While any core of the IIB-VIB, IIIB-VB or IVB-IVB semiconductors can beused, the core must be such that, upon combination with a cap, aluminescent quantum dot results. A IIB-VIB semiconductor is a compoundthat contains at least one element from Group IIB and at least oneelement from Group VIB of the periodic table, and so on. Preferably, thecore is a IIB-VIB, IIIB-VB or IVB-IVB semiconductor that ranges in sizefrom about 1 nm to about 10 nm. The core is more preferably a IIB-VIBsemiconductor and ranges in size from about 2 nm to about 5 nm. Mostpreferably, the core is CdS or CdSe. In this regard, CdSe is especiallypreferred as the core, in particular at a size of about 4.2 nm.

The “cap” is a semiconductor that differs from the semiconductor of thecore and binds to the core, thereby forming a surface layer on the core.The cap must be such that, upon combination with a given semiconductorcore, a luminescent quantum dot results. The cap should passivate thecore by having a higher band gap than the core. In this regard, the capis preferably a IIB-VIB semiconductor of high band gap. More preferably,the cap is ZnS or CdS. Most preferably, the cap is ZnS. In particular,the cap is preferably ZnS when the core is CdSe or CdS and the cap ispreferably CdS when the core is CdSe.

The “attachment group” as that term is used herein refers to any organicgroup that can be attached, such as by any stable physical or chemicalassociation, to the surface of the cap of the luminescent semiconductorquantum dot and can render the quantum dot water-soluble withoutrendering the quantum dot no longer luminescent. Accordingly, theattachment group comprises a hydrophilic moiety. Preferably, theattachment group enables the hydrophilic quantum dot to remain insolution for at least about one hour. More preferably the attachmentgroup enables the hydrophilic quantum dot to remain in solution for atleast about one day. Even more preferably, the attachment group allowsthe hydrophilic quantum dot to remain in solution for at least about oneweek, most preferably for at least about one month. Desirably, theattachment group is attached to the cap by covalent bonding and isattached to the cap in such a manner that the hydrophilic moiety isexposed. Preferably, the hydrophilic attachment group is attached to thequantum dot via a sulfur atom. More preferably, the hydrophilicattachment group is an organic group comprising a sulfur atom and atleast one hydrophilic attachment group. Suitable hydrophilic attachmentgroups include, for example, a carboxylic acid or salt thereof, asulfonic acid or salt thereof, a sulfamic acid or salt thereof, an aminosubstituent, a quaternary ammonium salt, and a hydroxy. The organicgroup of the hydrophilic attachment group is preferably a C₁-C₆ alkylgroup or an aryl group, more preferably a C₁-C₆ alkyl group, even morepreferably a C₁-C₃ alkyl group. Therefore, in a preferred embodiment,the attachment group of the present invention is a thiol carboxylic acidor thiol alcohol. More preferably, the attachment group is a thiolcarboxylic acid. Most preferably, the attachment group is mercaptoaceticacid.

A preferred embodiment of a water-soluble luminescent semiconductorquantum dot is one that comprises a CdSe core of about 4.2 nm in size, aZnS cap and an attachment group. Another preferred embodiment of awater-soluble luminescent semiconductor quantum dot is one thatcomprises a CdSe core, a ZnS cap, and a mercaptoacetic acid attachmentgroup. An especially preferred water-soluble luminescent semiconductorquantum dot comprises a CdSe core of about 4.2 nm, a ZnS cap of about 1nm, and a mercaptoacetic acid attachment group.

In another embodiment, a composition comprising a water-solubleluminescent semiconductor quantum dot as described above and an aqueouscarrier is provided. Any suitable aqueous carrier can be used in thecomposition. Desirably, the carrier renders the composition stable at adesired temperature, such as room temperature, and is of anapproximately neutral pH. Examples of suitable aqueous carriers areknown to those of ordinary skill in the art and include saline solutionand phosphate-buffered saline solution (PBS).

The luminescent material can comprise any of the above luminescentmaterials singly, or in a variety of combinations. For example, aplurality of different fluorescent materials can be used such that afirst fluorescent material absorbs and emits light at one set ofwavelengths and a second fluorescent material absorbs and emits light atanother set of wavelengths different from the first fluorescentmaterial. Alternatively, the flake could contain a first luminescentlayer, which is light-excited, and a second luminescent layer, which iscomposed of an electroluminescent material. One skilled in the art willrecognize, in view of the disclosure herein, that a wide variety ofluminescent materials and combinations thereof could be combined tocreate greatly enhanced effects.

When a distinct luminescent layer is formed in the flake structure, theluminescent layer has a thickness of about 50 Å to about 15,000 Å, morepreferably from about 50 Å to about 5,000 Å, and most preferably fromabout 50 Å to about 2500 Å. The aspect ratio of the flake structure witha distinct luminescent layer is preferably greater than about two. Theaspect ratio of the flakes is ascertained by taking the ratio of thelongest planar dimension of the flakes to the edge thickness dimensionof the flakes.

By incorporating the luminescent material within a multilayer thin filmstructure itself, there is no tendency for the luminescent andcolor-shifting materials to segregate. In addition, adding theluminescent material inside the flake or foil makes it very difficult todetect the presence of the luminescent material using optical orelectron microscopy.

One presently preferred method of fabricating a plurality of luminescentpigment flakes, each of which having the multilayer thin film coatingstructure of flake 20, is based on conventional web coating techniquesused to make optical thin films. Accordingly, an absorber layer isdeposited on a web of flexible material such as polyethyleneterephthalate (PET) which has an optional release layer thereon. Theabsorber layer can be formed by a conventional deposition process suchas PVD, CVD, PECVD, sputtering, or the like. The above mentioneddeposition methods enable the formation of a discrete and uniform layerabsorber layer of a desired thickness.

Next, a first dielectric layer, for example, is deposited on theabsorber layer to a desired optical thickness by a conventionaldeposition process. The dielectric layer is formed from a luminescentmaterial or a combination of luminescent and non-luminescent materials.The deposition of the dielectric layer can be accomplished by a vacuumdeposition process (e.g., PVD, CVD, PECVD).

The luminescent dielectric layer is exposed to the proper temperatureand atmospheric conditions to allow conversion of the luminescentmaterial to the proper stoichiometry. Alternatively, reactive gases canbe introduced into a PVD chamber during deposition to controlstoichiometry. For example, monatomic or diatomic oxygen can be used tocontrol oxidation. The vacuum deposition processes have the advantagethat they may be used with the high temperature processes required tomake materials luminescent. Vacuum deposition also allows for smoothnessand thickness control.

The reflector layer is then deposited on the first dielectric layer,taking on the characteristics of the underlying dielectric layer. Thisis followed by a second dielectric layer being deposited on thereflector layer and preferably having the same optical thickness as thefirst dielectric layer. The second dielectric layer may or may notinclude a luminescent material. Finally, a second absorber layer isdeposited on the second dielectric layer and preferably has the samephysical thickness as the first absorber layer.

Thereafter, the flexible web is removed, either by dissolution in apreselected liquid or by way of a release layer, both of which are wellknown to those skilled in the art. As a result, a plurality of flakesare fractured out during removal of the web from the multilayer thinfilm. This method of manufacturing pigment flakes is similar to thatmore fully described in U.S. Pat. No. 5,135,812 to Phillips et al., thedisclosure of which is incorporated by reference herein. The pigmentflakes can be further fragmented if desired by, for example, grindingthe flakes to a desired size using an air grind, such that the pigmentflakes have a dimension on any surface thereof ranging from about 2microns to about 200 microns.

While methods of depositing continuous thin film phosphorescentmaterials are known, few are broadly suitable for all the multilayerthin film structures disclosed and taught by the instant application.The ideal deposition process to produce highly efficient phosphors mustresult in crystalline films having exceptionally smooth surfacesnecessary for interference phenomena in multilayer films. Specifically,the deposition of thin film phosphors with the necessary degree ofcrystallinity during the deposition process produces a microstructurethat does not replicate the underlying substrate with sufficientregularity to be useful in most interference-based thin film designs.Since the crystalline morphology is not easily controlled, the resultantsurface roughness disallows further processing steps required in thinfilm interference-based designs. The most successful methods, such asliquid phase epitaxy or molecular beam deposition, only depositcrystalline thin films at extremely slow rates and hence are generallynot economically viable for the broadest applications of multilayer thinfilm structures.

A preferred deposition process that overcomes the deficiencies of priormethods is set forth in copending U.S. application Ser. No. 09/532,391,filed on Mar. 22, 2000, the disclosure of which is incorporated byreference herein. This patent teaches the use of thermal evaporation, ahigh deposition rate economical process, for producing thin filmphosphors of high crystallinity with the sufficiently smooth surfacesnecessary to function in an optical interference pigment flake or foil.It will be readily apparent to one of ordinary skill in the art of thinfilm technology that this process can be used to create flakes bydepositing the phosphor on a substrate or web material compatible withthe deposition and annealing temperature, such as glass, fused silica,or a stainless steel belt. If it is not desirable to coat other layerson the same substrate or web material, then flake-like pigment particlesare produced by removal of the thin film from the substrate or webmaterial. The phosphorescent flakes can be easily removed from thesubstrate or web material by providing therebetween a poorly adheringintermediate coating, such as a water soluble salt or thin layer ofmetal, which can be dissolved in a weak acid or can cause the flakes tobe removed by mechanical deformation of a flexible metal web when used.These phosphorescent pigment flakes may then be coated by subsequentprocesses such as sol-gel coating, electroplating, or CVD in a fluidizedbed. An alternative thin film deposition process applicable to themanufacture of phosphorescent crystalline flakes is disclosed in U.S.Pat. No. 6,025,677 to Moss et al., the disclosure of which isincorporated by reference herein.

In an alternative embodiment of a luminescent color-shifting flake 20,an asymmetrical color-shifting flake can be provided which includes athree-layer thin film stack structure with the same layers as on oneside of the reflector layer in flake 20 as shown in FIG. 1. Accordingly,the asymmetrical color-shifting flake includes a reflector layer, adielectric layer on the reflector layer, and an absorber layer on thedielectric layer. Each of these layers can be composed of the samematerials and have the same thicknesses as described above for thecorresponding layers of the above discussed flake 20. In addition,asymmetrical luminescent color-shifting flakes can be formed by a webcoating process such as described above in which the various layers aresequentially deposited on a web material to form a thin film structure,which is subsequently fractured and removed from the web to form aplurality of flakes.

FIGS. 2 and 3 are graphical representations of a color-shiftingmultilayered structure 30 of the invention which contains a luminescentlayer therein. The multilayered structure 30 includes a reflector layer32, a first dielectric layer 34, a luminescent layer 36, a seconddielectric layer 38, and an absorber layer 40. Although the luminescentlayer is illustrated as interposed between the dielectric layers, theluminescent layer can be positioned anywhere in the multilayeredstructure, so long as its optical characteristics are consistent withthose of the other layers in creating a color-shifting structure. Thus,the luminescent layer can optionally act as a dielectric, an absorber,or a reflector, depending on its location and optical characteristics.

FIGS. 2 and 3 also plot the theoretical electric field intensity 42relative to the distance an electromagnetic wave travels through amultilayer structure at zero degrees from normal. As illustrated inFIGS. 2 and 3, the luminescent layer 36 can be positioned anywherebetween reflector layer 32 and absorber layer 40. Thus, the luminescentlayer can be positioned in the center of the dielectric layers 34, 38(FIG. 2), substantially to one side (FIG. 3), or even completely to oneside and adjacent to the absorber or reflector layers (not shown). Inthose instances where luminescent layer 36 is incorporated betweendielectric layers 34, 38, the luminescent layer functions as adielectric and necessarily has dielectric qualities such as transparencyover most wavelength ranges. Where luminescent layer 36 is adjacent toeither reflector layer 32 or absorber layer 40, luminescent layer 36 canbe formed to function as a dielectric, an absorber, or a reflector andhave the requisite characteristics for such purposes. In this way, theoptical distance traveled by wavelengths incident upon and emitted fromthe luminescent layer 36 can be effectively controlled. As illustratedin FIG. 3, when the luminescent layer is configured to correspond to apeak electric field intensity 44, heightened luminescent effects areobserved.

One aspect of the invention is to match the excitation wavelength of theluminescent material with the position of the luminescent layer in thestack. By placing the luminescent layer in the right positioncorresponding to a maximum electric field at the excitation wavelengthof the luminescent layer, the efficiency of the luminescent emission isincreased. Conversely, the thickness of the luminescent layer can bedecreased through this optimum placement in the stack. This isadvantageous because if the luminescent material has unwantedabsorption, it would be advantageous to minimize the thickness of theluminescent layer in the stack.

The reflectance (and transmittance) spectra of thin film interferencedevices shifts to shorter wavelengths as the incident angle increasesaway from normal to the surface. The change in incident angle alsoshifts the electric field distribution within the thin film layers.Hence, the wavelength of maximum electric field intensity is a functionof incident angle. Therefore, if the luminescent layer is placed at theelectric field maximum at normal angle, then the intensity of theluminescent layer's absorption of the excitation energy should decreasewith increasing angle making the device an angle-sensitive luminescentdevice. If instead the excitation wavelength is designed to be at awavelength longer than that corresponding to the normal angle, thenchanging the angle will cause the absorption (and consequently theemission) intensity to start low, increase, then decrease again.

Thus, the multilayered structure shown in FIG. 2 would have anincreasing intensity with increasing angle, and the multilayeredstructure shown in FIG. 3 would have a decreasing intensity withincreasing angle.

Referring now to FIGS. 4-13, one skilled in the art will recognize, inview of the disclosure herein, that various luminescent materials asdiscussed above can be incorporated into the multilayered thin filmstructures discussed hereafter.

FIG. 4 depicts alternative coating structures (with phantom lines) for acolor-shifting pigment flake 50 in the form of an encapsulate accordingto other embodiments of the invention. The flake 50 has a core layer 52,which can be overcoated by an encapsulating dielectric layer 54substantially surrounding the core layer 52. An absorber layer 56, whichovercoats dielectric layer 54, provides an outer encapsulation of flake50. The hemispherical lines on one side of the flake 50 indicate thatdielectric layer 54 and absorber layer 56 can be formed as contiguouslayers.

Alternatively, core layer 52 and dielectric layer 54 can be in the formof a thin film core flake stack, in which opposing dielectric layers 54a, 54 b are preformed on the top and bottom surfaces but not on at leastone side surface of core layer 52, with the absorber layer 56encapsulating the thin film stack. An encapsulation process can also beused to form additional layers on flake 50 such as a capping layer (notshown).

Various luminescent materials as discussed above can be incorporatedinto the multilayer structure of flake 50 by the methods of theinvention. Suitable materials and thicknesses for the dielectriclayer(s) and absorber layer of flake 50 are the same as taughthereinabove for corresponding layers of flake 20 in FIG. 1. The corelayer 52 can comprise a metallic reflector such as discussed above forreflector layer 22 of flake 20, as well as other materials such glass,silica, mica, indium-tin-oxide (ITO), needles, micropatterned particles,liquid crystal platelets, and the like.

In addition, core layer 52 can be a multi-layered core flake sectionstructure, such as a “bright metal flake” as disclosed in U.S. Pat. No.6,013,370 to Coulter et al., and U.S. application Ser. No. 09/207,121,filed Dec. 7, 1998, now U.S. Pat. No. 6,150,022, the disclosures ofwhich are incorporated by reference herein. Such a multi-layeredstructure includes a reflector sublayer having a top surface, a bottomsurface, and at least one side surface, and a support sublayer preformedon at least one of the top and bottom surfaces but not on the at leastone side surface of the reflector sublayer. The reflector sublayer canbe a metal such as aluminum having a thickness of at least about 40 nm,and the support layer(s) can be a dielectric such as silicon oxidehaving a thickness of at least about 10 nm, with the thickness beingchosen so that the dielectric sublayers do not substantially affect thecolor properties of the reflector sublayer. For example, a multilayeredcore flake section can have the coating structure SiO_(x)/Al/SiO_(x),where x is from about 1 to about 2.

The core layer 52 can also be a multi-layered structure such as a“composite reflective flake” as disclosed in copending U.S. applicationSer. No. 09/626,041 to Coulter et al., filed Jul. 27, 2000, thedisclosure of which is incorporated by reference herein. Such amulti-layered structure includes a central support sublayer having a topsurface, a bottom surface, and at least one side surface, and areflector sublayer preformed on one or both of the top and bottomsurfaces but not on the at least one side surface of the reflectorsublayer.

FIG. 5 depicts another alternative coating structure for acolor-shifting pigment flake 60 according to the present invention. Theflake 60 includes a core layer 52 and a single dielectric layer 54,which extends over top and bottom surfaces of the core layer 52 to forma dielectric-coated preflake 62. The dielectric-coated preflake has twoside surfaces 64 and 66. Although side surface 66 is homogeneous andformed only of the dielectric material of dielectric layer 54, sidesurface 64 has distinct surface regions of dielectric 64 a, reflector 64b, and dielectric 64 c, respectively. The dielectric-coated preflake isfurther coated on all sides with an absorber layer 56. The absorberlayer is in contact with the dielectric layer 54 and core layer 52 atside surface 64. Various luminescent materials as described previouslycan be incorporated into the multilayer structure of flake 60 accordingto the methods of the invention.

The structure of the pigment flake 60 typically occurs because of apreflake coating process such as disclosed in U.S. application Ser. No.09/512,116, filed on Feb. 24, 2000, now abandoned, the disclosure ofwhich is incorporated by reference herein. In such a process, one ormore thin film layers including at least a core reflector layer aredeposited on a web to form a film, which is subsequently fractured andremoved from the web to form a plurality of pigment preflakes. Thepreflakes can be a dielectric-coated flake, in which a dielectriccoating completely encapsulates a core flake section. The preflakes arebroken into sized preflakes using any conventional fragmentationprocess, such as by grinding. The sized preflakes will include somesized preflakes having top and bottom dielectric layers with nodielectric material overcoating the side surfaces of the preflake, suchas shown for one embodiment of flake 50 in FIG. 4 in which the corelayer is coated with top and bottom dielectric layers. Other sizedpreflakes will have a single dielectric layer extending over both topand bottom surfaces of the core flake section, leaving one side surfaceof the core flake section exposed, such as shown for dielectric-coatedpreflake 62 in FIG. 5. Because of the fragmentation process,substantially all of the sized preflakes have at least a portion of aside surface exposed. The sized preflakes are then coated on all sideswith an absorber layer, such as shown for flake 60 of FIG. 5.

FIG. 6 depicts another alternative coating structure for acolor-shifting pigment flake 80 in the form of an encapsulate. The flakehas a thin core layer 82, which can be formed of a particulate substratematerial that provides rigidity, such as mica, glass flake, talc, orother silicatic material, as well as iron oxide, boron nitride, and thelike. The core layer 82 is overcoated on all sides with a reflectorcoating 84, such as a reflective metallic coating, which can be composedof the same materials as described above for reflector layer 22 of flake20. An encapsulating dielectric layer 54 substantially surroundsreflector coating 84. An absorber layer 56, which overcoats dielectriclayer 54, provides an outer encapsulation of flake 80. Variousluminescent materials as described previously can be incorporated intothe multilayer structure of flake 80 according to the methods of theinvention.

Various coating processes can be utilized in forming the dielectric andadsorber coating layers by encapsulation. For example, suitablepreferred methods for forming the dielectric layer include vacuum vapordeposition, sol-gel hydrolysis, CVD in a fluidized bed, andelectrochemical deposition. A suitable TiO₂ sol-gel process is describedin U.S. Pat. No. 5,858,078 to Andes et al., the disclosure of which isincorporated by reference herein. Other examples of suitable sol-gelcoating techniques useful in the present invention are disclosed in U.S.Pat. No. 4,756,771 to Brodalla et al., the disclosure of which isincorporated by reference herein; Zink et al., Optical Probes andProperties of Aluminosilicate Glasses Prepared by the Sol-Gel Method,Polym. Mater. Sci. Eng., 61, pp. 204-208 (1989); and McKiernan et al.,Luminescence and Laser Action of Coumarin Dyes Doped in Silicate andAluminosilicate Glasses Prepared by the Sol-Gel Technique, J. Inorg.Organomet. Polym., 1(1), pp. 87-103 (1991).

Suitable preferred methods for forming the absorber layers includevacuum vapor deposition, and sputtering onto a mechanically vibratingbed of particles, such as disclosed in U.S. application Ser. No.09/389,962, filed Sep. 3, 1999, now U.S. Pat. No. 6,241,858, which isincorporated by reference herein. Alternatively, the absorber coatingmay be deposited by decomposition through pyrolysis of metal-organocompounds of related CVD processes which may be carried out in afluidized bed as described in U.S. Pat. Nos. 5,364,467 and 5,763,086 toSchmid et al., the disclosures of which are incorporated by referenceherein. Another method of depositing the absorbers of the invention isby plasma enhanced chemical vapor deposition (PECVD) where the chemicalspecies are activated by a plasma. Such a method is disclosed in detailin U.S. application Ser. No. 09/685,468, filed on Oct. 10, 2000, whichis incorporated by reference herein.

If no further grinding is carried out, these encapsulation methodsresult in an encapsulated core flake section with dielectric andabsorber materials therearound. Various combinations of the abovecoating processes may be utilized during manufacture of pigment flakeswith multiple encapsulating coatings. When pigment flakes are formed bya sequential encapsulation process, it will be appreciated that eachrespective encapsulating layer is generally a continuous layer composedof one material and having substantially the same thickness around theflake structure.

There are various methods which can be utilized to incorporate aluminescent material or layer into a color-shifting flake formed byencapsulation processes. Including luminescent materials in amultilayered color-shifting particle is difficult because of the hightemperatures required to process the luminescent materials and obtainthe proper stoichiometry. The present invention provides various methodssuch that the stoichiometry problem can be overcome.

In one method, thin film speed particles are coated with luminescentpre-cursors in a sol-gel or equivalent process, and then the particlesare heated in the appropriate atmosphere in order to create the properstoichiometry. For example, the luminescent material is first dissolvedor suspended in a sol-gel precursor solution and absorbed into pores ofthe sol-gel coating or bound to the surface of the coating. The sol-gelsolution is then deposited onto the surfaces of seed particles throughmethods known in the art, for example, by solution codeposition. Next,the speed particles are removed from the sol-gel solution. Finally,water and/or alcohol are removed from the sol-gel coating on theparticles by heating.

Alternatively, another method involves using a nanoreactor to providethe luminescent material in a matrix. This is accomplished by firstforming porous nanospheres from a sol-gel precursor solution. Thenanospheres are then imbibed with a luminescent material. Theluminescent-imbibed nanospheres are then deposited onto seed particles.Finally, the coated seed particles are separated from the precursorsolution. This method allows for control of the thickness andcomposition of the luminescent material.

In both of the above sol-gel methods, the seed particle can comprise amultilayer structure such as bright metal flake or preflake, or amonolithic structure. Further, these methods can be used to apply theluminescent material as a separate layer or as part of a reflector,dielectric, absorber, or other functional layer. The sol-gel methodshave the advantage of economics and efficiency, as well as ease ofprocess. They are compatible with fragile luminescent materials andallow for tailored materials with exact stoichiometries.

Further, these methods allow for ease in producing high luminescentmaterial doping levels, and provide for durable luminescent layersbecause the sol-gel coating shields the luminescent materials. Thesemethods also allow for ease of particle orientation control. The uniquecrystallography produced by these methods can lead to unique opticalproperties, and the unique refractive indices produced allow for controlof absorption.

FIG. 7 depicts a luminescent color-shifting pigment flake 100 accordingto another embodiment of the invention. The flake 100 is a multi-layerdesign having a generally symmetrical multilayer thin film stackstructure on opposing sides of a reflector layer 102. Thus, first andsecond dielectric layers 104 a, 104 b are disposed respectively onopposing sides of reflector layer 102, and first and second absorberlayers 106 a, 106 b are disposed respectively on each of the dielectriclayers 104 a, 104 b. A third dielectric layer 108 a is formed on thefirst absorber layer 106 a, and a fourth dielectric layer 108 b isformed on the second absorber layer 106 b. A third absorber layer 110 ais on the third dielectric layer 108 a, and a fourth absorber layer 110b is on the fourth dielectric layer 108 b. These layers of flake 100 canbe formed by a web coating and flake removal process as describedpreviously.

As shown in FIG. 7, each dielectric and absorber layer pair forms arepeating period 112, 114, of dielectric/absorber (e.g., layers 104 aand 106 a, and layers 108 a and 110 a). One or more additional periodsof dielectric/absorber layers may be added to flake 100 to obtain adesired optical effect.

FIG. 7 further shows an alternative coating structure (with phantomlines) for the luminescent color-shifting flake 100, in which one ormore of the absorber layers and dielectric layers are coated aroundreflector layer 102 in an encapsulation process. For example, when anencapsulation process is used to form an outer absorber layer, absorberlayers 110 a and 110 b are formed as part of a continuous absorbercoating layer 110 substantially surrounding the flake structurethereunder. Likewise, an encapsulation process can also be used informing an underlying dielectric layer, such that dielectric layers 108a and 108 b are formed as part of a continuous dielectric coating layer108 substantially surrounding the flake structure thereunder. Anencapsulation process can also be used in forming the other dielectric104 and absorber 106 layers such that reflector layer 102 isencapsulated sequentially with alternating dielectric and absorberlayers.

Thus, the pigment flake 100 may be embodied either as a multilayer thinfilm stack flake or a multilayer thin film encapsulated particle withone or more encapsulating layers therearound. Suitable materials andthicknesses for the absorber, dielectric, and reflector layers of flake100 are the same as taught hereinabove for flake 20. Various luminescentmaterials as described previously can be incorporated into themultilayer structure of flake 100 according to the methods of theinvention.

FIG. 8 depicts a luminescent color-shifting pigment flake 120 accordingto another embodiment of the invention which does not use a reflector.The flake 120 is a three-layer design having a generally symmetricalmultilayer thin film structure on opposing sides of a dielectric layer122. Thus, first and second absorber layers 124 a and 124 b are formedon opposing major surfaces of dielectric layer 122. These layers offlake 120 can be formed by a web coating and flake removal process asdescribed previously.

FIG. 8 further depicts an alternative coating structure (with phantomlines) for the luminescent color-shifting flake 120, in which anabsorber layer is coated around dielectric layer 122 in an encapsulationprocess. Accordingly, absorber layers 124 a and 124 b are formed as partof a continuous absorber coating layer 124 substantially surroundingdielectric layer 122.

Thus, the pigment flake 120 may be embodied either as a multilayer thinfilm stack flake or a multilayer thin film encapsulated particle.Suitable materials and thicknesses for the absorber and dielectriclayers of flake are the same as taught hereinabove for flake 20. Variousluminescent materials as described previously can be incorporated intothe multilayer structure of flake 120 according to the methods of theinvention.

FIG. 9 illustrates a pigment flake 130 according to a further embodimentof the present invention. Pigment flake 130 comprises a core layer 132which is substantially encapsulated by a first absorber layer 134. Theabsorber layer 134 is in turn encapsulated by a dielectric layer 136,such as a layer of SiO₂ or high index TiO₂ formed by a sol-gel process.A second absorber layer 138 encapsulates dielectric layer 136. Thus,pigment flake 130 is embodied as a multilayer thin film encapsulatedparticle. The core layer 132 is preferably a flat, transparent planarmaterial such as mica, glass, silica, indium tin oxide (ITO), or otherdielectric material, which gives strength to the flake. Suitablematerials and thicknesses for the absorber layers of flake 130 are thesame as taught hereinabove for flake 20. Various luminescent materialsas described previously can be incorporated into the multilayerstructure of flake 130 according to the methods of the invention.

Some flakes of the invention can be characterized as multilayer thinfilm interference structures in which layers lie in parallel planes suchthat the flakes have first and second parallel planar outer surfaces andan edge thickness perpendicular to the first and second parallel planarouter surfaces. Such flakes are produced to have an aspect ratio of atleast about 2:1, and preferably about 5-15:1 with a narrow particle sizedistribution. The aspect ratio of the flakes is ascertained by takingthe ratio of the longest planar dimension of the first and second outersurfaces to the edge thickness dimension of the flakes.

The luminescent color-shifting pigment flakes of the present inventioncan be interspersed within a pigment medium to produce a colorantmaterial which can be applied to a wide variety of objects or papers.The pigment flakes added to a medium produces a predetermined opticalresponse through radiation incident on a surface of the solidifiedmedium. Suitable pigment media include various polymeric materials ororganic binders such as acrylic melamine, urethanes, polyesters, vinylresins, acrylates, methyl methacrylate, ABS resins, epoxies, styrenes,ink and paint formulations based on alkyd resins, and mixtures thereof.The luminescent color-shifting flakes combined with the pigment mediaproduce a colorant material that can be used directly as a paint, ink,or moldable plastic material. The colorant material can also be utilizedas an additive to conventional paint, ink, or plastic materials.

In addition, the luminescent color-shifting flakes can be optionallyblended with various additive materials such as conventional pigmentflakes, particles, or dyes of different hues, chroma and brightness toachieve the color characteristics desired. For example, the flakes canbe mixed with other conventional pigments, either of the interferencetype or noninterference type, to produce a range of other colors. Thispreblended material can then be dispersed into a polymeric medium suchas a paint, ink, plastic or other polymeric pigment vehicle for use in aconventional manner.

Examples of suitable additive materials that can be combined with theluminescent color-shifting flakes of the invention includenon-color-shifting high chroma or high reflective platelets whichproduce unique color effects, such as MgF₂/Al/MgF₂ platelets orSiO₂/Al/SiO₂ platelets. Other suitable additives that can be mixed withthe luminescent color-shifting flakes include lamellar pigments such asaluminum flakes, graphite flakes, glass flakes, iron oxide, boronnitride, mica flakes, interference based TiO₂ coated mica flakes,interference pigments based on multiple coated plate-like silicaticsubstrates, metal-dielectric or all-dielectric interference pigments,and the like; and non-lamellar pigments such as aluminum powder, carbonblack, ultramarine blue, cobalt based pigments, organic pigments ordyes, rutile or spinel based inorganic pigments, naturally occurringpigments, inorganic pigments such as luminescent dioxide, talc, chinaclay, and the like; as well as various mixtures thereof. For example,pigments such as aluminum powder or carbon black can be added to controllightness and other color properties.

The luminescent color-shifting flakes of the present invention areparticularly suited for use in applications where colorants of highchroma and durability are desired. By using the luminescentcolor-shifting flakes in a colorant material, high chroma durable paintor ink can be produced in which variable color effects are noticeable tothe human eye. The luminescent color-shifting flakes of the inventionhave a wide range of color-shifting properties, including large shiftsin chroma (degree of color purity) and also large shifts in hue(relative color) with a varying angle of view. Thus, an object coloredwith a paint containing the luminescent color-shifting flakes of theinvention will change color depending upon variations in the viewingangle or the angle of the object relative to the viewing eye.

The luminescent color-shifting flakes of the invention can be easily andeconomically utilized in paints and inks which can be applied to variousobjects or papers, such as motorized vehicles, currency and securitydocuments, household appliances, architectural structures, flooring,fabrics, sporting goods, electronic packaging/housing, productpackaging, etc. The luminescent color-shifting flakes can also beutilized in forming colored plastic materials, coating materials,extrusions, electrostatic coatings, glass, and ceramic materials.

Generally, the color-shifting foils of the invention have anonsymmetrical thin film coating structure, which can correspond to thelayer structures on one side of a core layer in any of the abovedescribed embodiments related to thin film stack flakes. For example, afoil can be formed with repeating dielectric/absorber periods on oneside of a reflector layer such as shown for the flake in FIG. 7. Variousluminescent materials as described previously can be incorporated intothe multilayer structure of the foils described as follows according tothe methods of the invention. The foils can be laminated to variousobjects or can be formed on a carrier substrate.

FIG. 10 depicts a coating structure of a luminescent color-shifting foil200 formed on a substrate 202, which can be any suitable material suchas a flexible PET web, carrier substrate, or other plastic material. Asuitable thickness for substrate 202 is, for example, about 0.5 to about7 mils. The foil 200 includes a reflector layer 204 on substrate 202, adielectric layer 206 on reflector layer 204, and an absorber layer 208on dielectric layer 206. The reflector, dielectric and absorber layerscan be composed of the same materials and can have the same thicknessesas described above for the corresponding layers in flake 20.

The foil 200 can be formed by a web coating process, with the variouslayers as described above sequentially deposited on a web byconventional deposition techniques to form a thin film foil structure.The foil 200 can be formed on a release layer (not shown) of a web sothat the foil can be subsequently removed and attached to a surface ofan object. The foil 200 can also be formed on a carrier substrate 202,which can be a web without a release layer.

FIG. 11 depicts a coating structure of a color-shifting foil 210 formedon a carrier substrate 212. The foil 210 includes a first absorber layer214 on substrate 212, a dielectric layer 216 on absorber layer 214, anda second absorber layer 218 on dielectric layer 216, but does notinclude a reflector layer. Such a film structure allows the foil to betransparent to light incident upon the surface thereof, therebyproviding for visual verification or machine readability of informationbelow foil 210 on carrier substrate 212. The dielectric and absorberlayers of foil 210 can be composed of the same materials and can havethe same thicknesses as described above for the corresponding layers inflake 20.

The foils of the invention can be used in a hot stamping configurationwhere the thin film stack of the foil is removed from the release layerof a substrate by use of a heat activated adhesive. The adhesive can beeither coated on a surface of the foil opposite from the substrate, orcan be applied in the form of a UV activated adhesive to the surface onwhich the foil will be affixed. Further details of making and usingoptical stacks as hot stamping foils can be found in U.S. Pat. Nos.5,648,165, 5,002,312, 4,930,866, 4,838,648, 4,779,898, and 4,705,300,the disclosures of which are incorporated by reference herein.

FIG. 12 illustrates one embodiment of a foil 220 disposed on a web 222having an optional release layer 224 on which is deposited a reflectorlayer 226, a dielectric layer 228, and an absorber layer 230. The foil220 may be utilized attached to web 222 as a carrier when the releaselayer is not employed. Alternatively, foil 220 may be laminated to atransparent substrate (not shown) via an optional adhesive layer 232,such as a transparent adhesive or ultraviolet (UV) curable adhesive,when the release layer is used. The adhesive layer 232 can be applied toabsorber layer 230 or the transparent substrate.

FIG. 13 depicts an alternative embodiment in which a foil 240 having thesame thin film layers as foil 220 discussed above is disposed on a web222 having an optional release layer 224. The foil 240 is formed suchthat absorber layer 230 is deposited on web 222. The foil 240 may beutilized attached to web 222 as a carrier, which is preferablytransparent, when the release layer is not employed. The foil 240 mayalso be attached to a substrate (not shown) when release layer 224 isused, via an adhesive layer 242 such as a hot stampable adhesive, apressure sensitive adhesive, a permanent adhesive, and the like. Theadhesive layer 242 is applied to reflector layer 226 or can be appliedto the substrate.

As discussed hereinabove, it is desirable to add additional covertfeatures to color-shifting devices. Accordingly, it has beenadvantageously discovered that the luminescent color-shifting pigmentsof the present invention can effectively be used in a mixture withdistinct luminescent color-shifting pigments or non-luminescentcolor-shifting pigments in varying predetermined ratios to add covertfeatures to color-shifting pigment compositions. The covert featureprovides a particular advantage in packaging and sales of thecolor-shifting pigments to customers because the covert feature allowsthe manufacturer to track their products based upon the customer to whomit is sold.

Thus, a luminescent color-shifting pigment can comprise a mixture ofcolor-shifting pigments and luminescent color-shifting pigments. Forexample, a product with a preferred color shift can be sold in a mixtureof about 80% of the color-shifting pigment and about 20% of theluminescent color-shifting pigment to one customer. Another productcould be sold to a different customer with about 60% of thenon-luminescent color-shifting pigment and about 40% of the luminescentcolor-shifting pigment. Although the pigments will have virtuallyidentical color-shifting features, the differing percentages ofluminescent pigments will create two products with differing magnitudesof luminescence as a covert feature. Therefore, while only manufacturingtwo color-shifting pigment products that have the same color shift (oneluminescent), a variety of distinguishable color-shifting devices can beproduced by varying the ratio of the two pigments in the mixture toproduce color-shifting compositions that have varying predetermineddegrees of luminescence.

Accordingly, another embodiment of the invention is directed to acolorant composition which includes luminescent and non-luminescentcolor shifting pigment flakes mixed in a predetermined ratio anddispersed in a pigment medium. In one preferred embodiment, theluminescent and non-luminescent color-shifting pigment flakes arecombined in a mixture comprising at least about 1 wt-% luminescentcolor-shifting pigment flakes prior to dispersing in a pigment medium.In a more preferred embodiment, the luminescent and non-luminescentcolor-shifting pigment flakes are combined in a mixture comprising atleast about 10 wt-% luminescent color-shifting pigment flakes prior todispersing in a pigment medium.

Because the varying percentages of luminescent color-shifting pigment ismeasurable, a manufacturer or distributor of the product can track theuse of the product they have sold to individual customers to insurecompliance with sales contracts. The covert feature can also be used inanti-counterfeiting measures for various products and documents. Thecovert feature allows the manufacture of the luminescent andnon-luminescent color-shifting pigments to distinguish individualcolorant compositions even though they may be indistinguishable to thoseunaware of the covert feature.

Of course one skilled in the art will recognize, in light of thedisclosure herein, that multiple varieties of non-luminescent andluminescent color-shifting flakes may be combined to vary and increasethe number of covert features in the ink. Thus, two or more distinctluminescent color-shifting pigments can be combined in a mixture with anon-luminescent color-shifting pigment. All of the pigments can have thesame color shift with different luminescence effects. This approachprovides the ability to create unique luminescent signatures by mixingcolor-shifting pigments. For example, one could mix 60% of anon-luminescent pigment, 30% of a luminescent pigment type A, and 10% ofa luminescent pigment type B, with the resulting admixture having acharacteristic luminescent signature that was the average of 30% A and10% B while still having the desired overt color-shifting performance.To obtain a new luminescent signature, one could simply vary the ratioof type A to type B rather than synthesize new pigments.

Another way to distinguish pigments with the same color shift is to havevarying depths of luminescent layers within the multilayer structure ofthe pigments, thus creating pigments with differing angle sensitiveluminescence and providing yet another layer of covert security. Forexample, one luminescent pigment may be designed to have a maximumluminescence under light incident at 45° to normal while an otherwiseidentical pigment could be designed to have a maximum luminescence underlight incident at 60° to normal.

The present invention provides numerous advantages and benefits.Primarily, the present invention provides pigment or foil components forsecurity devices which have distinct and pronounced overt and covertfeatures under visible wavelength and non-visible wavelength lightsources. These distinct features are not easily duplicated and cannot becopied by simple mixtures of interference pigments and luminescentmaterials. Another advantage is that the covert features are machinereadable, yet coexist with overt features such as the color-shiftingnature of the materials. Thus, although it may be apparent that securityfeatures exist, it is not apparent how many security features arepresent. Further, the covert features can selectively code additionalinformation.

The luminescence feature adds to the value of color-shifting pigmentproducts by potentially increasing thermal stability, mechanicalstability, and durability. This feature also provides light fastness, aswell as solvent and moisture resistance. Additionally, polarizationcontrol can be achieved by both the control of luminescent materialorientation in the flake and the control of flake orientation in a resinor coating composition. The luminescence feature also providesadvantages with regard to its spectral rectifying effects, such asangularly dependent luminescence or wavelength selective quenching.

Incorporating luminescent materials into multilayer flakes also hasadvantages over mixtures of luminescent particles and color-shiftingflakes as to the shape of the luminescent materials. These advantagesprincipally go to the “lay down” of the flakes. In other words, thegeometric positioning of the luminescent material in a flake is flat,allowing for uniform positioning and thus uniform orientation of theluminescent flakes. Also, the inherent shape of the flake can be used tocontrol the morphology of a luminescent layer in the flake and therebyprovide for new optics.

The following examples are given to illustrate the present invention,and are not intended to limit the scope of the invention.

EXAMPLE 1

FIG. 14 is a graph illustrating the way in which absorption varies in aluminescent color-shifting thin film stack by angle of incidence, andthe characteristic absorption of a luminescent material at 520 nm. Thereflectance of a luminescent thin film stack is plotted against theincident wavelength. Three separate hypothetical absorption curves 300,302, and 304 for light incident respectively at 45°, 0°, and 60° fromnormal are plotted. A reflector/dielectric/absorber coating structurehaving nominal gold-to-green color shift, with a luminescent materialplaced within the dielectric layer, is hypothetically considered. Inthis example, the luminescent material placed within the dielectriclayer absorbs at 520 nm and re-emits at 720 nm. This difference inwavelength between the apex of the absorption and emission spectrums iscalled the Stokes shift.

The notch 306 in the 45° curve illustrates the characteristic absorptionof the luminescent material at 520 nm. As can be seen, the 45° incidentlight will be absorbed by the luminescent material and reemitted at 720nm, while light incident at 60° or 0° will generally not be absorbed bythe luminescent material. Such a characteristic allows a luminescentmaterial to be selected to emit outside of observation wavelengths whileabsorbing within the observation wavelengths. This creates an effect inwhich there are absorption bands detected at certain angles but notothers, providing a covert taggant detectable in the spectrum.

EXAMPLE 2

FIG. 15 is a graph illustrating the angle-sensitive emission 320 of aluminescent color-shifting pigment according to the invention. The graphillustrates how changes in the incident angle of electromagnetic energyresults in different emission levels. As shown, there exists a peak 322of maximum absorption that corresponds to a particular wavelength. Thus,a given luminescent will highly absorb at one angle of incidence but notat others. This feature of the invention allows for furthercustomization and differentiation of luminescent color-shifting pigmentsand foils.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed and desired to be secured by United States LettersPatent is:
 1. A luminescent color-shifting pigment flake, comprising: areflector layer; a first dielectric layer overlying the reflector layer;a first absorber layer overlying the first dielectric layer; and atleast one luminescent material incorporated into the pigment flake;wherein the pigment flake exhibits a discrete color shift such that thepigment flake has a first color at a first angle of incident light orviewing and a second color different from the first color at a secondangle of incident light or viewing.
 2. The pigment flake of claim 1,further comprising a second dielectric layer overlying the reflectorlayer opposite from the first dielectric layer.
 3. The pigment flake ofclaim 2, further comprising a second absorber layer overlying the seconddielectric layer opposite from the first absorber layer, the secondabsorber layer having a physical thickness of about 30 Å to about 300 Å.4. The pigment flake of claim 1, wherein the absorber layersubstantially surrounds the dielectric layer and the reflector layer,and the dielectric layer substantially surrounds the reflector layer. 5.The pigment flake of claim 2, wherein the absorber layer substantiallysurrounds the first and second dielectric layers and the reflectorlayer.
 6. The pigment flake of claim 1, wherein the reflector layercomprises a reflective material selected from the group consisting ofaluminum, silver, copper, gold, platinum, tin, titanium, palladium,nickel, cobalt, rhodium, niobium, chromium, and combinations or alloysthereof.
 7. The pigment flake of claim 1, wherein the reflector layerhas a physical thickness of about 200 Å to about 10,000 Å.
 8. Thepigment flake of claim 1, wherein the reflector layer comprises a coreflake section including a reflector sublayer having a top surface, abottom surface, and at least one side surface, and a support sublayerpreformed on at least one of the top and bottom surfaces but not on theat least one side surface of the reflector sublayer.
 9. The pigmentflake of claim 1, wherein the reflector layer comprises a compositereflective flake including a central support sublayer having a topsurface, a bottom surface, and at least one side surface, and areflector sublayer preformed on each of the top and bottom surfaces butnot on the at least one side surface of the reflector sublayer.
 10. Thepigment flake of claim 2, wherein the first and second dielectric layerscomprise a dielectric material having an index of refraction of about1.65 or less.
 11. The pigment flake of claim 10, wherein the dielectricmaterial is selected from the group consisting of silicon dioxide,aluminum oxide, magnesium fluoride, aluminum fluoride, cerium fluoride,lanthanum fluoride, neodymium fluoride, samarium fluoride, bariumfluoride, calcium fluoride, lithium fluoride, and combinations thereof.12. The pigment flake of claim 2, wherein the first and seconddielectric layers comprise a dielectric material having an index ofrefraction of greater than about 1.65.
 13. The pigment flake of claim12, wherein the dielectric material is selected from the groupconsisting of zinc sulfide, zinc oxide, zirconium oxide, titaniumdioxide, carbon, indium oxide, indium-tin-oxide, tantalum pentoxide,cerium oxide, yttrium oxide, europium oxide, iron oxides, hafniumnitride, hafnium carbide, hafnium oxide, lanthanum oxide, magnesiumoxide, neodymium oxide, praseodymium oxide, samarium oxide, antimonytrioxide, silicon carbide, silicon nitride, silicon monoxide, seleniumtrioxide, tin oxide, tungsten trioxide, and combinations thereof. 14.The pigment flake of claim 2, wherein the first and second dielectriclayers have an optical thickness in a range from about 2 QWOT at adesign wavelength of about 400 nm to about 9 QWOT at a design wavelengthof about 700 nm.
 15. The pigment flake of claim 2, wherein the first andsecond dielectric layers have the same optical thickness.
 16. Thepigment flake of claim 2, wherein the first and second dielectric layersare composed of the same material.
 17. The pigment flake of claim 2,wherein the first and second dielectric layers are each composed of adielectric optical stack having a plurality of alternating layers of ahigh index material and a low index material.
 18. The pigment flake ofclaim 17, wherein the dielectric optical stack has a gradient index ofrefraction.
 19. The pigment flake of claim 2, wherein the first andsecond dielectric layers are each composed of a mixture or multiplesublayers of dielectric materials selected from the group consisting oflow index materials, high index materials, and combinations thereof. 20.The pigment flake of claim 3, wherein the first and second absorberlayers comprise an absorbing material selected from the group consistingof chromium, nickel, aluminum, palladium, platinum, titanium, vanadium,cobalt, iron, tin, tungsten, molybdenum, rhodium, niobium, carbon,graphite, silicon, germanium, and compounds, mixtures, or alloysthereof.
 21. The pigment flake of claim 3, wherein the first and secondabsorber layers comprise a material selected from the group consistingof elemental titanium, titanium-based compounds, titanium-based alloys,and combinations thereof.
 22. The pigment flake of claim 3, wherein thefirst absorber layer has a physical thickness of about 30 Å to about 300Å.
 23. The pigment flake of claim 3, wherein the first and secondabsorber layers have the same physical thickness.
 24. The pigment flakeof claim 3, wherein the first and second absorber layers are composed ofthe same material.
 25. The pigment flake of claim 1, wherein theluminescent material is excited by one or more energy sources selectedfrom the group consisting of infrared radiation, ultraviolet radiation,visible light, electric fields, magnetic fields, and chemical reaction.26. The pigment flake of claim 1, wherein the luminescent materialexhibits fluorescence or phosphorescence.
 27. The pigment flake of claim1, wherein the luminescent material comprises a fluorescent dye combinedwith a liquid crystal polymer.
 28. The pigment flake of claim 1, whereinthe luminescent material comprises a crystalline phosphor material. 29.The pigment flake of claim 1, wherein the luminescent material comprisessolid phase or water-soluble quantum dot particles.
 30. The pigmentflake of claim 1, wherein the luminescent material is incorporated intothe flake as a distinct luminescent layer having a physical thickness ofabout 50 Å to about 15,000 Å.
 31. The pigment flake of claim 1, whereinthe pigment flake exhibits a luminescence intensity that is dependentupon the angle at which incident light at excitation wavelengths entersthe pigment flake.
 32. A luminescent color-shifting pigment materialcomprising a plurality of color-shifting pigment flakes, the pigmentflakes having a multilayer structure as defined in claim
 1. 33. Aluminescent color-shifting colorant composition, comprising: a pigmentmedium; and a plurality of luminescent color-shifting pigment flakesdispersed in the pigment medium, the pigment flakes having a multilayerstructure as defined in claim
 1. 34. The colorant composition of claim33, wherein the pigment medium comprises a material selected from thegroup consisting of acrylic melamine, urethanes, polyesters, vinylresins, acrylates, methyl methacrylate, ABS resins, epoxies, styrenes,ink and paint formulations based on alkyd resins, and mixtures thereof.35. The colorant composition of claim 33, wherein the pigment flakeshave a dimension on any surface thereof ranging from about 2 microns toabout 200 microns.
 36. The colorant composition of claim 33, furthercomprising a plurality of non-luminescent color-shifting pigment flakesdispersed in the pigment medium.
 37. The colorant composition of claim36, wherein the non-luminescent color-shifting pigment flakes and theluminescent color-shifting pigment flakes are combined in a mixturecomprising at least about 1 wt-% luminescent color-shifting pigmentflakes prior to being dispersed in the pigment medium.
 38. The colorantcomposition of claim 36, wherein the plurality of luminescentcolor-shifting pigment flakes include two or more luminescent flaketypes having the same color shift and combined in a predetermined ratioto produce a selected luminescent signature for the colorantcomposition.
 39. A luminescent color-shifting pigment flake, comprising:a first absorber layer; a first dielectric layer overlying the firstabsorber layer; a reflector layer overlying the first dielectric layer;a second dielectric layer overlying the reflector layer; a secondabsorber layer overlying the second dielectric layer; and at least oneluminescent material incorporated into the pigment flake; wherein thepigment flake exhibits a discrete color shift such that the pigmentflake has a first color at a first angle of incident light or viewingand a second color different from the first color at a second angle ofincident light or viewing.
 40. The pigment flake of claim 39, furthercomprising a third dielectric layer overlying the second absorber layer,and a fourth dielectric layer underlying the first absorber layer. 41.The pigment flake of claim 40, wherein the third and fourth dielectriclayers form a continuous coating layer around the layers interiorthereto.
 42. The pigment flake of claim 40, further comprising a thirdabsorber layer overlying the third dielectric layer, and a fourthabsorber layer overlying the fourth dielectric layer.
 43. The pigmentflake of claim 42, wherein the third and fourth absorber layers form acontinuous coating layer around the layers interior thereto.
 44. Thepigment flake of claim 42, wherein the pigment flake exhibits aluminescence intensity that is dependent upon the angle at whichincident light at excitation wavelengths enters the pigment flake. 45.The pigment flake of claim 39, wherein at least one of the firstdielectric layer and the second dielectric layer is composed partiallyor wholly of the luminescent material.
 46. The pigment flake of claim39, wherein at least one of the first absorber layer and the secondabsorber layer is composed partially or wholly of the luminescentmaterial.
 47. The pigment flake of claim 39, wherein the luminescentmaterial is incorporated into the flake as a distinct luminescent layerhaving a physical thickness of about 50 Å to about 15,000 Å.
 48. Aluminescent color-shifting pigment flake, comprising: a core layer; adielectric layer substantially surrounding the core layer; an absorberlayer substantially surrounding the dielectric layer; and at least oneluminescent material incorporated into the pigment flake; wherein thepigment flake exhibits a discrete color shift such that the pigmentflake has a first color at a first angle of incident light or viewingand a second color different from the first color at a second angle ofincident light or viewing.
 49. The pigment flake of claim 48, furthercomprising at least one additional dielectric layer and at least oneadditional absorber layer which substantially surround the other layersof the pigment flake.
 50. The pigment flake of claim 48, wherein thepigment flake exhibits a luminescence intensity that is dependent uponthe angle at which incident light at excitation wavelengths enters thepigment flake.
 51. The pigment flake of claim 48, wherein the dielectriclayer is composed partially or wholly of the luminescent material. 52.The pigment flake of claim 48, wherein the absorber layer is composedpartially or wholly of the luminescent material.
 53. The pigment flakeof claim 48, wherein the luminescent material is incorporated into theflake as a distinct luminescent layer having a physical thickness ofabout 50 Å to about 15,000 Å.
 54. A luminescent color-shifting pigmentflake, comprising: a core reflector layer having a top surface, a bottomsurface, and at least one side surface; a dielectric layer overlying thetop surface and the bottom surface but not on at least one side surfaceof the reflector layer; an absorber layer substantially surrounding thedielectric layer and in contact with at least one side surface of thereflector layer; and at least one luminescent material incorporated intothe pigment flake; wherein the pigment flake exhibits a discrete colorshift such that the pigment flake has a first color at a first angle ofincident light or viewing and a second color different from the firstcolor at a second angle of incident light or viewing.
 55. The pigmentflake of claim 54, wherein the pigment flake exhibits a luminescenceintensity that is dependent upon the angle at which incident light atexcitation wavelengths enters the pigment flake.
 56. The pigment flakeof claim 54, wherein the dielectric layer is composed partially orwholly of the luminescent material.
 57. The pigment flake of claim 54,wherein the absorber layer is composed partially or wholly of theluminescent material.
 58. The pigment flake of claim 54, wherein theluminescent material is incorporated into the flake as a distinctluminescent layer having a physical thickness of about 50 Å to about15,000 Å.
 59. A luminescent color-shifting pigment flake, comprising: acore layer; a reflector coating substantially surrounding the corelayer; a dielectric layer substantially surrounding the reflectorcoating; an absorber layer substantially surrounding the dielectriclayer; and at least one luminescent material incorporated into thepigment flake; wherein the pigment flake exhibits a discrete color shiftsuch that the pigment flake has a first color at a first angle ofincident light or viewing and a second color different from the firstcolor at a second angle of incident light or viewing.
 60. The pigmentflake of claim 59, wherein the core layer comprises a material selectedfrom the group consisting of mica, glass, talc, iron oxide, and boronnitride.
 61. The pigment flake of claim 59, wherein the pigment flakeexhibits a luminescence intensity that is dependent upon the angle atwhich incident light at excitation wavelengths enters the pigment flake.62. The pigment flake of claim 59, wherein the dielectric layer iscomposed partially or wholly of the luminescent material.
 63. Thepigment flake of claim 59, wherein the absorber layer is composedpartially or wholly of the luminescent material.
 64. The pigment flakeof claim 59, wherein the luminescent material is incorporated into theflake as a distinct luminescent layer having a physical thickness ofabout 50 Å to about 15,000 Å.
 65. A luminescent color-shifting pigmentflake, comprising: a core layer; a first absorber layer substantiallysurrounding the core layer; a dielectric layer substantially surroundingthe first absorber layer; a second absorber layer substantiallysurrounding the dielectric layer; and at least one luminescent materialincorporated into the pigment flake; wherein the pigment flake exhibitsa discrete color shift such that the pigment flake has a first color ata first angle of incident light or viewing and a second color differentfrom the first color at a second angle of incident light or viewing. 66.The pigment flake of claim 65, wherein the core layer is composed ofmica or glass.
 67. The pigment flake of claim 65, wherein the dielectriclayer is composed of silicon dioxide or titanium dioxide.
 68. Thepigment flake of claim 67, wherein the dielectric layer is formed by asol-gel process.
 69. The pigment flake of claim 65, wherein the pigmentflake exhibits a luminescence intensity that is dependent upon the angleat which incident light at excitation wavelengths enters the pigmentflake.
 70. The pigment flake of claim 65, wherein the dielectric layerincludes the luminescent material.
 71. The pigment flake of claim 65,wherein at least one of the first and second absorber layers is composedpartially or wholly of the luminescent material.
 72. The pigment flakeof claim 65, wherein the luminescent material is incorporated into theflake as a distinct luminescent layer having a physical thickness ofabout 50 Å to about 15,000 Å.
 73. A luminescent color-shifting pigmentflake, comprising: a first absorber layer; a dielectric layer overlyingthe first absorber layer; a second absorber layer overlying thedielectric layer; and at least one luminescent material incorporatedinto the pigment flake; wherein the pigment flake exhibits a discretecolor shift such that the pigment flake has a first color at a firstangle of incident light or viewing and a second color different from thefirst color at a second angle of incident light or viewing.
 74. Thepigment flake of claim 73, wherein the pigment flake exhibits aluminescence intensity that is dependent upon the angle at whichincident light at excitation wavelengths enters the pigment flake. 75.The pigment flake of claim 73, wherein the dielectric layer includes theluminescent material.
 76. The pigment flake of claim 73, wherein atleast one of the first and second absorber layers is composed partiallyor wholly of the luminescent material.
 77. The pigment flake of claim73, wherein the luminescent material is incorporated into the flake as adistinct luminescent layer having a physical thickness of about 50 Å toabout 15,000 Å.
 78. The pigment flake of claim 73, wherein the first andsecond absorber layers form a continuous coating layer that encapsulatesthe dielectric layer.
 79. A luminescent color-shifting foil, comprising:a reflector layer; a dielectric layer overlying the reflector layer; anabsorber layer overlying the dielectric layer; and one or moreluminescent materials incorporated into the foil; wherein the foilexhibits a discrete color shift such that the foil has a first color ata first angle of incident light or viewing and a second color differentfrom the first color at a second angle of incident light or viewing. 80.The foil of claim 79, further comprising an adhesive layer forlaminating the foil to a substrate.
 81. The foil of claim 80, whereinthe adhesive layer is selected from the group consisting of a hotstampable adhesive, a pressure sensitive adhesive, a permanent adhesive,a transparent adhesive, and a UV curable adhesive.
 82. The foil of claim79, wherein the luminescent materials are excited by one or more energysources selected from the group consisting of infrared radiation,ultraviolet radiation, visible light, electric fields, magnetic fields,and chemical reaction.
 83. The foil of claim 79, wherein the luminescentmaterials exhibit fluorescence or phosphorescence.
 84. The foil of claim79, wherein the luminescent materials are composed of solid phase orwater-soluble quantum dot particles.
 85. The foil of claim 79, whereinthe foil exhibits a luminescence intensity that is dependent upon theangle at which incident light at excitation wavelengths enters the foil.86. The foil of claim 79, wherein the dielectric layer includes aluminescent material.
 87. The foil of claim 79, wherein the absorberlayer includes a luminescent material.
 88. The foil of claim 79, whereinthe luminescent material is incorporated into the foil as a distinctluminescent layer having a physical thickness of about 50 Å to about15,000 Å.
 89. A color-shifting foil device, comprising: a carriersubstrate; a first absorber layer overlying the carrier substrate; adielectric layer overlying the first absorber layer; a second absorberlayer overlying the dielectric layer; and one or more luminescentmaterials incorporated into the foil; wherein the foil exhibits adiscrete color shift such that the foil has a first color at a firstangle of incident light or viewing and a second color different from thefirst color at a second angle of incident light or viewing.
 90. A methodof fabricating a luminescent pigment flake or foil material thatexhibits a discrete color shift such that the material has a first colorat a first angle of incident light or viewing and a second colordifferent from the first color at a second angle of incident light orviewing, the method comprising: providing one or more reflector layers;forming one or more dielectric layers over the reflector layers; formingone or more absorber layers over the dielectric layers; andincorporating a luminescent material into the flake or foil as aseparate layer or as at least part of one or more of the reflectorlayer, dielectric layer, or absorber layer.
 91. The method of claim 90,wherein the dielectric and absorber layers are formed by a processselected from the group consisting of physical vapor deposition,chemical vapor deposition, plasma enhanced chemical vapor deposition,sputtering, and electrolysis deposition.
 92. The method of claim 90,wherein the luminescent material is excited by one or more energysources selected from the group consisting of infrared radiation,ultraviolet radiation, visible light, electric fields, magnetic fields,and chemical reaction.
 93. The method of claim 90, wherein theluminescent material exhibits fluorescence or phosphorescence.
 94. Themethod of claim 90, wherein the luminescent material comprises solidphase or water-soluble quantum dot particles.
 95. The method of claim90, wherein the luminescent material is incorporated into the flake orfoil by a sol-gel process.
 96. The method of claim 90, wherein theluminescent material is dissolved in a sol-gel precursor solution priorto being incorporated into the flake or foil.
 97. The method of claim95, wherein the sol-gel process comprises: forming porous nanospheresfrom a sol-gel precursor solution; imbibing the nanospheres with aluminescent material; and depositing the luminescent-imbibed nanospheresonto one or more of the layers, or to form at least part of one or moreof the layers in the flake or foil material.